1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) wireless communication system.
2. Description of the Related Art
A difference in propagation delay time of a multi-path propagation path cannot be neglected when a frequency bandwidth used in communication is expanded to enhance a wireless communication speed. In an environment of incoming signals having the propagation delay times, a waveform distortion caused by inter-code interference becomes a main factor of deterioration of communication quality. In such environments, an Orthogonal Frequency Division Multiplexing (hereinafter abbreviated as OFDM) system is well known as a system in which the waveform distortion caused by the inter-code interference can be compensated even if the signals having the propagation delay times are received.
Because a signal becomes a complex signal in the OFDM transmission scheme, it is necessary that a quadrature modulator and a quadrature demodulator be used in a transmitter and a receiver respectively. In order to correctly produce an in-phase component signal and an orthogonal component signal during transmission, it is necessary to produce the signals whose phases are shifted from each other by 90 degrees. In order to correctly extract the in-phase component signal and the orthogonal component signal during reception, it is necessary that local signals whose phases are shifted from each other by 90 degrees be produced to multiply the local signals by the reception signals respectively. In the case where a gain is applied to the in-phase component signal and the orthogonal component signal by a filter or an amplifier, it is necessary to apply the equal gain to the in-phase component and the orthogonal component. However, generally a difference in amplitude between the in-phase component and the orthogonal component, or a phase error is generated in a 90-degree phase shifter. Hereinafter such incompleteness of an analog circuit is referred to as IQ imbalance.
When the IQ imbalance is generated, two subcarrier signals located on a frequency axis in symmetrical relation to a center frequency interfere with each other in plural subcarriers of the OFDM signal, which largely restricts transmission performance. Therefore, for example, JP-A 2001-119364 (KOKAI) proposes a technique in which an interference amount of the subcarriers symmetrically located in relation to the center frequency is estimated and compensation is performed based on the estimate result.
However, in the technique disclosed in JP-A 2001-119364 (KOKAI), because the interference amount in which both the IQ imbalance and a channel characteristic are combined is estimated, an influence of the individual IQ imbalance generated by the quadrature modulation cannot be learned. Therefore, the estimate processing is required in each frame, in the case where signals are received from unspecified number terminals or in the case where a channel fluctuation cannot be neglected. Additionally, in the conventional technique, because the influence of the individual IQ imbalance generated by the quadrature modulation cannot be learned, compensation of the IQ imbalance generated by the quadrature modulation cannot be performed to the transmission signal.
Additionally, in the conventional technique, a channel estimating known signal having a length double usually necessary length be transmitted to all the subcarriers in which data is transmitted, which results in a problem of increased overhead.